This invention relates to a method of selectively removing zinc from an acid solution, in particular from pickling baths as used in galvanizing plants. These acid based pickling baths comprise a high content of zinc ions and iron ions, while, moreover, traces of other metal ions are present.
Hot dip galvanizing is a method in which the surface of steel structures is treated and provided with a layer of zinc. This method comprises a dipping process in which the total structure is immersed in a bath of liquid zinc. Three types of hot dip galvanizing methods can be distinguished, namely dry galvanizing, wet galvanizing and continuous galvanizing. In case of continuous galvanizing steel is drawn directly from the rolling mill through a galvanizing bath. Before it is galvanized, the rolled steel is passed through a reducing atmosphere. In this process no use is made of pickling baths so that the present invention is not applicable to this continuous galvanizing process.
Wet galvanizing is a technique especially used in the United States of America. Dry galvanizing is mostly used in Europe.
Both techniques in general comprise the following process steps. A material or structure to be galvanized is degreased in a suitable bath at a temperature of about 70-80.degree. C., often followed by rinsing the degreased product. Then pickling is carried out in a solution of a strong acid, preferably hydrochloric acid, although, for instance, sulfuric acid and phosphoric acid may be used as well. After the pickling step rinsing in general is also carried out. In order to avoid oxidation in the air or other undesirable processes, the pickled material is fluxed. This means that the material is immersed in a solution containing one or more protective salts. This solution may have both ambient temperature and a higher temperature of, e.g., 70.degree. C. After immersion in the flux bath the material is dried, with the protective salts remaining on the material. Finally, the material can be galvanized. In a frequently used embodiment this is done by hot dip galvanizing at about 450.degree. C. An alternative is, for instance, high temperature galvanizing in which the galvanizing step is performed at a temperature of about 530-570.degree. C. or more.
Structural steel is generally supplied in the form of bright structural steel possessing a rolling skin and/or being rusted. The rolling skin is formed during hot rolling of the steel and is constituted by a hardly removable oxide layer. Rusting takes place under atmospheric conditions at both high and low temperature. Moreover, bright structural steel is protected in some cases by a thin layer of preserving oil intended to prevent rust from being formed during storage and transport. This layer of oil can be removed, for instance, by alkaline degreasing.
Degreasing is generally performed by a dipping operation in an alkaline solution or by a spraying operation in which an alkaline solution is sprayed onto the material to be galvanized. Besides, the alkaline degreasing step can be replaced by other degreasing steps known to those skilled in the art, e.g., by degreasing with acid or neutral cleaning.
After degreasing the oxide layers must be removed. The above rolling skin consists of FeO, Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4 and passes into rust during prolonged storage. By pickling the steel this iron oxide solution is removed. Pickling usually comprises the treatment of the steel with a concentrated hydrochloric acid solution, although sulfuric acid and phosphoric acid baths may be used as well. The pickling baths comprise inhibitors or pickling inhibitors for preventing steel from being affected by the acid from the pickling bath. Usually, tertiary and secondary amines are applied as inhibitors.
When hydrochloric acid solutions are used in the pickling bath, the concentration of HCl should not be too high. Emission of hydrogen chloride in the form of hydrochloric acid vapors must be avoided, at least be limited.
The fluxing of the pickled steel has two important functions. In the first place, fluxing prevents rust from being rapidly formed (rust film). The second function of flux salts is the function as a flux so that the zinc can better interact with the steel surface.
The difference between wet and dry galvanizing only resides in the manner of fluxing. In case of wet galvanizing a flux layer floats like a foam blanket on the zinc bath containing liquid zinc. In case of dry galvanizing a flux bath is placed before the zinc bath. This flux bath may have ambient temperature or a temperature of about 70.degree. C. After removal of the steel structure from a hot flux bath the water evaporates; when a cold flux bath is used, the flux must be dried in a drying oven. In both cases a flux layer is formed before it arrives at the zinc bath. In case of wet galvanizing a temperature gradient is present over the foam blanket so that especially thin material heats up more gradually and becomes warped less rapidly.
In case of dry galvanizing fluxing is usually carried out with a flux on the basis of a ZnCl.sub.2 /NH.sub.4 Cl mixture, which flux has a cleaning effect and a surface tension-reducing effect. The foam blanket used in the wet galvanizing process may comprise a glycerol NH.sub.4 Cl mixture.
Galvanizing takes place at a temperature of about 450.degree. C.; high temperature galvanizing at 530-600.degree. C. or more. In the bath containing molten zinc a number of additives is present to influence the layer thickness and to promote flowing off. The fluxed material is dipped into the zinc bath, maintained therein for a moment and then slowly drawn from the zinc bath. The layer thickness of the zinc on the steel is, as those skilled in the art know, determined by, inter alia, the temperature of the zinc bath, the dipping time, the reactivity of the steel and the composition of the zinc bath.
As stated above, the object of pickling is to free the steel of iron oxides, such as rolling skins that may be present. A pickling bath usually comprises a solution of concentrated hydrochloric acid to which pickling inhibitors and detergents have been added. In the course of time, the acid concentration in the pickling bath falls and the concentrations of iron, zinc and trace metal ions increase. As a result of the change in the composition of the pickling bath the pickling rate decreases and/or the bath becomes saturated, and in the long run the bath will have to be replaced.
The zinc ions mainly originate from stripping of already galvanized materials or structures rejected during quality control and of contaminated hoisting and hanging materials, e.g. traverse cells and cables. The other metal ions mainly originate from the steel to be pickled, from alloy layers and from the zinc bath, in which these metals are present as additives.
When the pickling capacity of the pickling bath becomes insufficient, the composition of the bath must be changed or replaced. Because of the high zinc content of the effluent then occurring, this product legally falls under chemical waste and/or can nowhere be delivered for processing. Therefore, the bath to be replaced is designated by the term "spent acid" or "saturated pickling acid".
In the last few years, the processing of this zinc-containing spent acid has increasingly caused problems. The transport and dumping of this waste product is more and more restrained by the authorities. Companies must look for alternatives in the form of reprocessing methods in order to reuse as many waste products as possible so that the effluent decreases.
It is an object of the present invention to solve or reduce the spent acid problem. In other words, it is an object: of the present invention to provide a method of removing zinc ions from a pickling bath used, in which the resulting flows can be reused as much as possible and the final effluent has the lowest possible volume.
As far as the composition is concerned, a pickling bath mainly comprises an aqueous solution in which up to about 150 g/l HCl is present in addition to Fe(II) ions and zinc ions in amounts up to maximally their saturation values. In use, the acid concentration in the bath is kept high by strengthening the bath with a concentrated hydrochloric acid solution (e.g. 30%) so that the chloride concentration in the bath will increase in the course of time. Besides, the pickling bath comprises traces of heavy metals (Mn, Cr, Ni, Cu, Pb, Sn, etc.) and small amounts of detergents and pickling inhibitors.
It is known that the removal of metals from a solution can be effected by using cation exchangers. It is difficult, however, to find a cation exchanger with which ions of a specific metal can be removed selectively, even if the conditions of exchange are adjusted critically. Moreover, in a very acid medium cation exchangers will nearly only be in the H.sup.+ form.
Based on the exchange of zinc ions, the absorbing capacity of cation exchangers for zinc ions from an iron ion-containing solution at a pH of 0.5-4.0 is relatively low. Besides, in many cases the absorbing capacity for zinc strongly depends on the pH, in which connection a good capacity is not obtained until at pH values of about 3. It is further observed that the presence of Fe.sup.2+ ions considerably decreases the capacity of the cation exchanger for zinc.